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rejected by all except Mexico, whose government had performed a volte-face. However, the nascent Martian nation declared itself in favor of the Nigerghanaians, pointing out that it could mount the project for a fraction of the cost any terrestrial nation would have incurred: launch prices from Mars were far smaller than those from Earth, and the asteroid belt, with its invaluable raw materials, was several tens of millions of kilometers nearer. When a historian dug out the idea of the Von Neumann probe—which had been popular among theoreticians centuries earlier, long before the human species had had the technological ability to construct any such thing—the Martians told the Earth nations that they could be part of the project, or not.
    The idea of the Von Neumann probe is a very simple one. If you can create a bot probe so sophisticated that it will guide itself to the vicinity of another star, it takes very little extra effort to make it capable of finding, in the orbit of that star, a random chunk of rock—an asteroid or a dead moon—on which it can set down and start constructing a replica of itself while at the same time making a survey of the stellar system and reporting home. The replica—or, if there's nothing interesting in this particular stellar system, both the parent and the offspring—can then head off towards different nearby stars. The enthusiasts for the concept had, throughout the period between the late twentieth century and the mid-twenty-first century, regarded this as the paradigmatic fashion in which any technological species would investigate the Universe. The idea fell from fashion when it became apparent that there were almost certainly no Von Neumann probes currently at work in the Solar System: had other civilizations hit on the idea there should, by the mere laws of statistics, have been plenty.
    What the Martians did was adapt the notion a little. In the middle of the twenty-fourth century they put a colony on Ceres and built five probes simultaneously; the effort strained Mars's revenues considerably, even though most of the nations of Earth provided contributory funding. Completed, the five probes were launched into the asteroid belt to discover, essentially, what they could eat. When, some years later, Hubble XVII was able to observe the first of the offspring blasting off in the general direction of Epsilon Eridani, there was widespread rejoicing on Mars. There would have been widespread rejoicing on Earth as well, except that it was in the middle of another global war: fortunately no nukes or micro-organisms were used, but it was a pity about the population of Patagonia.
    In 2510 Mars picked up the first signals from one of the cloned bot probes: Proxima Centauri was orbited by seven lifeless, atmosphereless lumps of rock, none of them larger than the Solar System's Mercury. No one was startled or disappointed by the news: Proxima, itself orbiting distantly around the binary of Alpha and Beta Centauri, had never been regarded as a hot prospect. Still, it would have been nice had the first probe report been positive.
    The second one was. It came in 2512, and it came from the system of Tau Ceti. Here there were only five planets orbiting the little star. The second one out had an atmosphere that was rather richer in oxygen than that of Mars, a gravity zero point eight three that of the Earth, and abundant vegetation. Whether or not humans would find it in fact habitable was something the probe could not determine: only human beings themselves could do that.
    By trying it out.
    It was a ruthless means of experimentation, but no one could think of a better one.
    So, on Phobos, the Martians began the construction of the Santa Maria , which would hold forty-five human beings for a thirty-year trip, and was capable of supporting them for a further eighty years if it proved obvious that Tau Ceti II was a complete non-starter: in that case they would explore the system, learning what they